2 EMBRYOLOGY. 



attention of biologists from the very earliest periods. Before the 

 establishment of the cell theory the origin of the organism from tbe 

 germ was not known to be an occurrence of the same nature as the 

 growth of the fully formed individual, and Embryological investiga- 

 tions were mixed up with irrelevant speculations on the origin of life 1 . 

 The difficulties of understanding the formation of the individual 

 from the structureless germ led anatomists at one time to accept 

 the view " according to which the embryo preexisted, even though 

 " invisible, in the ovum, and the changes which took place during 

 "incubation consisted not in a formation of parts, but in a growth, 

 "i.e. in an expansion with concomitant changes of the already 



" existing germ. 



Great as is the interest attaching to the simple and isolated life 

 histories of individual organisms, this interest has been increased 

 tenfold by the generalizations of Mr Charles Darwin. 



It has long been recognized that the embryos and larvre of the 

 higher forms of each group pass, in the course of their development, 

 through a series of stages in which they more or less completely 

 resemble the lower forms of the group 2 . This remarkable phenome- 

 non receives its explanation on Mr Darwin's theory of descent. 

 There are, according to this theory, two guiding, and in a certain sense 

 antagonistic principles which have rendered possible the present order 

 of the organic world. These are known as the laws of heredity 'and 

 variation. The first of these laws asserts that the characters of an 

 organism at all stages of its existence are reproduced in its descen- 

 dants at corresponding stages. The second of these laws asserts that 

 offspring never exactly resemble their parents. By the common 

 action of these two principles continuous variation from a parent type 

 becomes a possibility, since every acquired variation has a tendency 

 to be inherited. 



The remarkable law of development enunciated above, which has 

 been extended, especially by the researches of Huxley 3 and Kowalevsky, 

 beyond the limits of the more or less artificial groups created by 

 naturalists, to the whole animal kingdom, is a special case of the first 

 of the above laws. This law, interpreted in accordance with the 

 theory of descent, asserts that each organism in the course of its in- 

 dividual ontogeny repeats the history of its ancestral development. 

 It may be stated in another way so as to bring out its intimate con- 



1 To tins general statement Wolff forms a remarkable exception, for though without 

 any clear knowledge of what we call cells he had very distinct notions ou the relations 

 of growth and development. 



2 Von Baer who is often stated to have established the above generalization really 

 maintained a somewhat different view. He held (Ueber Entwicklungsgeschichte d. 

 Thiere, p. 224) that the embryos of higher forms never resembled the adult stages of 

 lower forms but merely the embryos of such forms. Von Baer was mistaken in thus 

 absolutely limiting the generalization, but his statement is much more nearly true than 

 a definite statement of the exact similarity of the embryos of higher forms to the 

 adults of lower ones. 



3 Huxley was the first to shew that the body of the Coelenterata was formed of two 

 layers, and to identify these with the two primary germinal layers of the Vertebrata. 



